1. Field of the Invention
The present invention relates to the provision of electrical isolation between two associated systems which operate at different power levels and particularly to the prevention of damage to a condition sensor should a fault occur at apparatus controlled in response to the condition sensed thereby. More specifically, the present invention is directed to an isolation amplifier which passes control signals, for example as provided by a sensor subassembly which operates at a low voltage, to the controlled apparatus, a high voltage operated pump or motor for example, while assuring against delivery of the high voltage to components of the sensor subassembly in the event of an electrical fault at the controlled apparatus. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
Isolation devices are used extensively in the process control and electric utility industries as buffers between hazardous and non-hazard locations and between safety and non-safety related areas. An example of a hazardous location would be one wherein a potentially explosive atmosphere occurs and condition responsive devices are employed to monitor that atmosphere. An example of safety versus non-safety areas would be a nuclear power plant wherein sensors positioned within the reactor core assembly provide information for use by external controllers and replacement of the sensor devices, should they be damaged by exposure to excess voltage or current fed back from a controller, would be exceedingly difficult at best.
An isolation device obviously must be characterized by a high degree of reliability. In the case of nuclear power plant usage, an isolation amplifier must meet the performance requirements set forth in IEEE Standard No. 279, Sections 3.7, 3.8 and 4.7.2. Isolation amplifiers have previously been available which satisfy the above-mentioned IEEE Standard criteria. However, as will be discussed below, these previously available isolation amplifiers have nevertheless had certain deficiencies.
In a typical operating environment, the function of an isolation amplifier is to prevent the high voltage used to energize a motor or other device from reaching the hazardous or safety related area should a fault condition occur through the device and its controls while supplying normal signals from a sensor subassembly located in the said area under non-fault conditions. Most previously available isolation amplifiers are permanently damaged by output fault voltages which are a fraction of that employed to energize the controlled device. Some previously available isolation amplifiers further permit a small amount of energy to be fed back from the controlled device to the amplifier input when a high voltage fault appears at the isolation amplifier output. Obviously, feed-through of energy from the output to the input of an isolator is undesirable. Also, the need to repair or replace an isolator after the occurrence of a fault should optimally be obviated. Thus, an ideal isolator will be characterized by no feed-through from the output to the input and by immediate recovery after the fault has been corrected.
Isolation amplifiers are used in a wide variety of applications. Thus, signal inputs to the isolation amplifier can be from many sources, can be either in analog or digital form and can be either voltage or current. Also, the power available for operating the isolation amplifier typically may be either from a 120 VAC source or a 12 VDC source. This wide variety of inputs, in combination with a potential choice of power sources, has resulted in many different isolation amplifiers being available to the protective system designer. System design and maintenance would be greatly facilitated if a single "off-the-shelf" isolation amplifier was available which, with minor modification commensurate with the intended use environment, could meet a wide variety of input and power source requirements.